Abstract
This study compares two methods of mobility-control design for chemical enhanced oil recovery processes. Method 1 matches the total relative fluid mobility upstream and downstream of the shock front. In method 2 the viscosity of the displacing agent is selected such that the total mobility at the shock water saturation is equal to or less than the minimum mobility across the saturation range. The two methods are based on fractional flow analysis of one-dimensional flow and they are validated against two-dimensional simulations of flow through heterogeneous permeable media. Our results emphasize the key role of the water/oil relative permeability curves for the design of mobility control in polymer and surfactant/polymer flooding. The polymer viscosity obtained by setting the shock-front mobility ratio to one (method 1) is the minimum viscosity to ensure a stable displacement front. Design by method 2 results in a larger viscosity than method 1. This shifts the shock water saturation to larger values and hence more oil is displaced. Moreover, we find that for surfactant-polymer (SP) solutions with ultra-low interfacial tension (IFT) reduction (Winsor type III), the required polymer viscosity is always greater than the oil viscosity (at low shear rates). However, for Winsor type I solutions, for oils with medium and large viscosity the non-linear shape of the relative permeability function leads to polymer viscosities that are less than that of the oil. For light oils the viscosity of the ASP solution should be significantly larger than the oil viscosity.
Highlights
Efficient extraction of oil from subsurface formations depends on (1) the displacement or microscopic sweep efficiency and (2) the macroscopic sweep efficiency, which for a homogeneous medium, depends on the stability of the displacement front (Bedrikovetsky, 1993; Lake et al, 2014)
It is assumed that when γow > 1 mN/m surfactant will have no effect on the oil recovery and the original water/oil relative permeability parameters are used
The Polymer Utilization Factor (PUF) is defined as the volume of the produced oil per mass of the injected polymer it is be used as proxy for more lengthy economic calculations
Summary
Efficient extraction of oil from subsurface formations depends on (1) the displacement or microscopic sweep efficiency (determined largely by the mobility ratio between the displacing agent and the in-situ fluids and reductions in the interfacial tension) and (2) the macroscopic sweep efficiency, which for a homogeneous medium, depends on the stability of the displacement front (Bedrikovetsky, 1993; Lake et al, 2014). For this case, the front remains stable; because the end-point mobility ratio (the ratio between the relative mobility of the displacing fluid at the injection point and the relative mobility of the oil at irreducible water saturation) is still greater than 1 fingers are initiated within the bank behind the front, but they do not reach the front, most likely because their growth is relative to the speed of the saturations within the finger, which propagate slower than the shock-front saturation. The addition of even small amounts of polymer will lead to an increase in oil recovery, compared to water or polymer solution with low viscosity, for maximum utilization of the injected polymer the displacement front should remain stable. We neglect the effect of relative permeability hysteresis in this study
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